1. Field of the Invention
This invention relates to a method of indirect liquid-phase heat transfer using as a heat transfer fluid in a high temperature, continuously circulating system a substantially anhydrous composition comprising a polyalkoxylated monohydric or polyhydric C.sub.5 to C.sub.36 aliphatic alcohol having a viscosity of less than about 100 centistokes, preferably from about 20 to about 60 centistokes at 100.degree. F.
2. Prior Art Discussion
Ideally, an indirect liquid phase heat transfer fluid suitable for use in a high temperature, continuously circulating system should provide a broad range of service temperatures, have a sufficiently low viscosity to provide an acceptable rate of heat transfer and minimized operating expenses, have a sufficiently low freezing point to reduce start-up and pumping problems at lower temperatures, have sufficient lubricity that stress on pumps, valves and other system parts is minimized, decompose at a desirably slow rate in use, and resist the formation of system-fouling degradation products. Moreover, for convenient handling, clean-up and disposal it is desirable that the compositions be environmentally non-hazardous and water-soluble.
Numerous compositions have been suggested for use as indirect liquid-phase heat transfer fluids. Exemplary of some of these compositions are petroleum oils; synthetic aromatic hydrocarbons such as the phenylene oxides and diphenylene oxides disclosed in U.S. Pat. No. 1,905,850, the terphenyls disclosed in U.S. Pat. No. 2,033,702, and the phenoxybiphenyls and phenoxyterphenyls disclosed in U.S. Pat. No. 3,957,666; and polyalkylene glycol type copolymers of ethylene oxide and propylene oxide having molecular weights of from about 400 to about 1,200, such as the polymers of UCON.RTM. HTF-500 heat transfer fluid available from Union Carbide Corporation, described in the product brochure entitled "UCON.RTM. Heat Transfer Fluid 500" published in 1981, and Poly-G WS-280X available from Olin Corporation, described in the product brochure entitled "Poly-G WS-280X Heat Transfer Fluid", published in 1982.
Of the previously cited fluids, petroleum oils are the least desirable. These compositions generally have rapid rates of thermal degradation, low flash points, high vapor pressures, low thermal efficiency, tend to form system-fouling varnishes and sludges as degradation products and, owing to their water insolubility and potential toxicity present clean-up and disposal problems.
Unlike petroleum oils and polyalkylene glycols, synthetic aromatics have the advantage of providing a broad range of service temperatures, oftentimes up to about 700.degree. F. However, these compositions have disadvantageously low flash points, (typically below operating temperatures, necessitating the use of pressurized systems capable of separating the fluid from air), present potential toxicity problems, and are relatively poor lubricants.
For systems operating at temperatures up to about 500.degree. F., water-soluble polyalkylene glycols stabilized by the addition thereto of at least one antioxidant are the indirect liquid-phase heat transfer fluids of choice. At these service temperatures polyalkylene glycols have low viscosities, relatively slow rates of thermal degradation, high thermal efficiency, excellent lubricity and resistance to the formation of system-fouling degradation products. At temperatures in excess of 500.degree. F., however, the previously cited polyalkylene glycols available as heat transfer fluids tend to decompose at relatively rapid rates.
The relative stability of a polymer in use as a heat transfer fluid may be considered in relation to viscosity changes as function of time at a given temperature. After a certain period of elevated temperature use petroleum oils, synthetic aromatic hydrocarbons and polyalkylene glycols all experience viscosity increases as a result of thermal and oxidative degradation, however, it is the rate of viscosity increase which limits the useful service life of a polymer at a particular temperature. The rate at which viscosity increases is also generally indicative of the thermal efficiency of a polymer, the coefficient of thermal conductivity varying approximately inversely with the square root of viscosity.
FIG. 1, attached hereto, is illustrative of the viscosity changes with time of UCON.RTM. HTF-500 heat transfer fluid, a 1-butanol initiated polyalkylene glycol having an average molecular weight of about 1100, and Therminol.RTM. 66 heat transfer fluid, a modified terphenyl, available from Monsanto Co., at both 500.degree. F. and 550.degree. F. as per the Thermal Stability Test defined in the section entitled "Examples" infra. At temperatures of about 500.degree. F. UCON.RTM. HTF-500 heat transfer fluid experiences initial viscosity decreases with time, whereas, Therminol.RTM. 66 heat transfer fluid continuously increases in viscosity at a relatively steady rate. As a general rule, fluids having viscosities of less than about 100 centistokes at 100.degree. F. normally require replacement when their viscosity in use increases by about 20 to 30 percent. At service temperatures of about 500.degree. F., UCON.RTM. HTF-500 heat transfer fluid experiences initial viscosity losses with the formation of some volatile, non-fouling degradation product. Until the fluid undergoes viscosity increasing degradation (i.e., some point in time beyond the scale depicted in FIG. 1) a system containing UCON.RTM. HTF-500 heat transfer fluid may be maintained by venting off volatiles and adding small amounts of replacement fluid as needed. In contrast thereto, Therminol.RTM. 66 heat transfer fluid, which cannot be so maintained, is generally used until sludge formation or viscosity increases necessitate complete fluid replacement and equipment cleanout. At temperatures of about 550.degree. F. the relatively rapid rate of thermal decomposition of UCON.RTM. HTF-500 heat transfer fluid obviates its lower temperature advantages.
Apart from the addition of antioxidants, other methods for improving the thermal stability of polyoxyalkylene-containing compounds have been suggested. Canadian Pat. No. 928,283 discloses that the thermal stability of polyoxyalkylene compounds may be increased by the incorporation of an amino group into the polymeric chain. However, at temperatures in excess of about 500.degree. F., nitrogen containing polyoxyalkylene compounds tend to decompose at undesirable rates and/or to form undesirable varnishes and/or sludges as degradation products (see Comparative Example C.sub.2).
The presence of an aromatic moiety has also been linked to the stability of polyokyalkylene-containing compounds. Alkoxylated alkyl phenols have been found to provide fluids having somewhat superior thermal stability, as compared to otherwise identical polyalkylene glycols lacking an aromatic moiety. Alkoxylated octyl phenols such as compositions available under the tradename Triton.RTM. X-100, available from Rohm and Haas Co., have been utilized as heat transfer fluids in solder blanketing operations, however, in continuously circulating systems such compositions generally develop undesirably high viscosities, typically, in excess of 100 Centistokes at 100.degree. F. (see Comparative Examples C.sub.5 and C.sub.6 for thermal stability data of ethoxylated nonyl phenols).
A heat transfer fluid having the thermal stability advantages exhibited by polyalkylene glycol type fluids at temperatures up to about 500.degree. F., useable at service temperatures in excess of about 500.degree. F. is highly desirable.